1. Field of the Invention
The invention relates to a technique, specifically apparatus and an accompanying method, for an optical scrambler and particularly one that provides scrambled states of polarization in an optical fiber.
2. Description of the Prior Art
Light emanating from many types of lasers is highly polarized and has a relatively constant state of polarization (SOP). The term SOP is a well-known defining metric which describes a relative position of electric fields that make up light and which remains unaltered unless encountering birefringence. Since there are many situations/applications where highly polarized light with a constant SOP is undesirable, optical scrambling was developed. Optical polarization scrambling effectively changes the SOP of light over time.
A technique has been developed in the art that ostensibly produces polarization-independent optical polarization scrambling. This technique utilizes a single-mode optical fiber that has been wrapped around a single cylindrical piezoelectric tube (PZT) where the tube is then activated with an electrical signal having a fixed frequency and a fixed amplitude. In practice, effective optical scrambling could only be achieved if the drive frequency and drive voltage supplied to the PZT were precisely adjusted and controlled. A major drawback of this conventional technique is that an input SOP had to be maintained at a particular SOP in order to achieve effective scrambling; therefore, this technique was actually polarization dependent. Adjusting the electrical drive voltage and frequency was problematic but having to control and maintain the input SOP was even more so and thus highly undesirable. Hence, this technique proved unworkable in practice.
Consequently, a goal still exists in the art to provide an optical scrambling technique which does not require that an input SOP be at any particular value and hence, by exhibiting substantial, if not complete, polarization-independence, obviating any need to control the input SOP.
Advantageously, the present invention overcomes the deficiencies associated with the conventional technique of providing optical polarization scrambling.
In accordance with my inventive teachings, polarization independence is achieved by wrapping a single optical fiber around each tube in a cascade of separate piezoelectric tubes(PZTs), with random amounts of fixed birefringence separating each tube, where each tube is then separately excited on a time-varying basis. Generally, a time-varying drive signal used to excite any one PZT is independent of that used to excite another such tube. Geometrical, physical displacement of each tube, resulting from its excitation, imparts a time-varying birefringence to that portion of the fiber wound around that particular tube. This time-varying birefringence perturbs an initial SOP of the light from its original pseudo-stationary position on a Poincarxc3xa9 sphere.
Specifically, in a preferred embodiment of the present invention, an optical signal, upon entering the scrambler, passes through a fiber that is tightly wrapped around each successive one of illustratively a first group of three PZTs. Each of these tubes is operated in a similar fashion and is illustratively excited by a modulated radio-frequency (RF) electrical drive signal. This signal, when applied to a PZT, causes that tube to exhibit geometric physical displacement, i.e., the tube slightly expands and contracts physically, in response to the signal. This time-varying displacement, which effectively stresses the fiber wound around that tube, induces time-varying birefringence in the fiber. This time-varying birefringence produced by each tube perturbs the initial SOP provided by that tube from its original pseudo-stationary position on the Poincarxc3xa9 sphere.
A preferred, though illustrative, modulation form of the RF drive signal uses combined frequency and amplitude modulation (FM/AM). Altering the number of independent signal sources by combining them may also be done but will result in slightly different optical performance benchmarks. Specifically, since each RF frequency delivered to a PZT, during FM modulation, requires a unique drive voltage for optimum scrambling, the amplitude of that signal is also varied, hence additionally imparting an amplitude modulation (AM) component to the FM drive signal. For any single FM/AM driven PZT, virtually all input SOPs experience reciprocating deflections from their initial statexe2x80x94from small 10xc2x0 arcs, to full rotations around the Poincarxc3xa9 sphere. Resulting direction and magnitude of the displacement around the Poincarxc3xa9 sphere depends on the input SOP, the drive voltage, and the RF frequency. For any one FM/AM driven PZT, only a few input SOPs are unaffected by that stage. For these few SOPs unaffected by the first stage, the light passes on through a fixed birefringence and then on to the next stage of the cascaded PZTs. The fixed inter-stage birefringence rotates the input SOP to a new and different SOP before entering the second FM/AM stage where the above process continues. Consequently, given these subsequent modulated stages, the probability of having a fixed non-deflecting SOP is greatly diminished. Empirical experience has shown that three such modulated stages, with their corresponding inter-stage fixed birefringences, provide an adequate level of scrambling for many applications but additional numbers of modulated stages (above three) provide even more random SOP movement. Since the drive frequency applied to each PZT, as a result of the frequency modulation, is such that the tube is driven at several of its resonant frequencies (at which power consumption peaks) for only a very short time, including those frequencies above and below these resonances, a benefit of this FM/AM technique is that advantageously a relatively low overall power is required to drive that tube than would otherwise occur had that tube be driven just at any of its resonant frequencies.
Once the optical signal passes through a final fixed birefringence, after passing through the last of the modulated stages, the SOP of that signal is now moving in a pseudo-random manner. At this point, the optical signal now moves on through the same fiber but which has been wrapped around group of illustratively three cascaded PZTs, with another corresponding set of inter-stage fixed birefringences. Each PZT in this latter cascaded group of tubes is illustratively shown as independently driven at a fixed frequency and a fixed amplitude. The frequency and amplitude of each of the electrical drive signals applied to the PZTs in this latter group can be different from each other. Similarly, altering the number of independent signal sources here by combining them may also be done but will result in slightly different optical performance benchmarks. Similarly to what occurs through the first cascaded group of PZTs, while exciting the second group of tubes with electrical signals of fixed frequency and amplitude, the electrical excitation causes a physical displacement of each of the PZT tubes in this second group, which, in turn, thereby imparts a further time-varying birefringence into the fiber and causes an additional change in the SOP.
I have found that a sufficient degree of polarization-independent scrambling can result from, as described, using six separate PZTs, with three cascaded PZTs located in each of two groups. Some level, though less, of polarization-independence will result from using fewer PZTs, while an enhanced level will result from using more PZTs. However, any marginal improvement in the polarization-independence that results from using each additional PZT over six is likely to diminish.